It is standard practice in the industrial production of glass sheets, that is window glass, mirrors, structural glass, or the like, to roughly cut the sheets slightly oversize, and then to grind-trim them down to exact dimensions. For this grinding the sheet is supported on a table with its sheet edges overhanging the table above a template having a template edge geometrically congruent to, and normally of the same dimensions as, the shape the sheet is to have after grinding. A grinder radially engages the edge of the sheet with a feeler in engagement with the template edge so that as the table and template are rotated about a vertical axis or the grinder is orbited about the table and template, the grinder grinds the rough-cut sheet on the table down to a size corresponding exactly to that of the template.
As a rule the rough-cut sheets are simply stacked up in a supply station adjacent the grinding station. A carriage horizontally displaceable on overhead tracks is provided with lifters, normally of the vacuum or suction type, that are dropped down onto the topmost sheet of the stack and lifted to raise it so this sheet can be transported horizontally and deposited on the work table. Such an arrangement is described in German published patent specification No. 2,756,443.
Unless the rough-cut sheets are exactly positioned in the supply station, which exact positioning is normally lost as sheets are picked off the top of the stack, and the transporter functions with great precision, normally necessitating very slow action to prevent the picked-up sheet from slipping laterally on the transporter, it is necessary for an operator in the grinding station to position the sheets exactly with respect to the template, that is to position them in vertical registration above the template. If the workpiece is horizontally offset in any direction it will be ground down too much on one edge and not at all on the opposite edge.
Such positioning is usually a manual task except in those machines where the transporter functions slowly and with enormous precision. Either way the deposition of the workpiece on the table and subsequent positioning of it robs valuable time from the production operation, time that slows down production.